Optical fibre transmission cables generally comprise one or more optical transmission fibres provided with one or more sheathings of synthetic resin material.
In addition, in order to provide the required mechanical strength, such cables are also provided with so-called strength members as a reinforcement.
More particularly, optical fibre transmission cables are inevitably subjected to tensile loads during the manufacture, handling, installation and, in some cases, the service life of the cables. For example, when such cables are strung in suspension from towers or the like, or when they are installed in ducts, it is necessary to exert substantial pulling forces on the cables. Also, strung cables are subject to tensile loads as a result of the weight of the cables themselves and also atmospheric conditions such as wind, ice etc.
Therefore, optical fibre transmission cables are provided with reinforcements to take up such loads in order to prevent breakage of the relatively weak optical fibres.
The provision of this reinforcement presents various difficulties. For example, if the reinforcement is required to be of an electrically non-conductive nature so as to avoid a danger of lightning strikes when the cables are freely suspended, then electrically conductive metal reinforcements such as aluminum and steel are not suitable for this purpose.
It has previously been proposed to provide, in an optical transmission cable, non-metallic strength members in the form of helically laid Kevlar aramid yarn, covered by a wrap of PTFE tape (see e.g. Modern Plastics, July 1978, pp. 38-41 and Design Engineering, March 1979). A jacket or sheathing of one or more layers of suitable material, e.g. polyethylene, is provided around the strength members.
However, a disadvantage of the use of Kevlar aramid yarn, which is a yarn made of highly oriented aramid fibre and is therefore flexible, and indeed a disadvantage of any flexible fibre yarn when used for optical fibre transmission cable reinforcement is that when it is provided with a sheathing of synthetic resin material, the latter shrinks as it is cured. This shrinkage causes the reinforcement to buckle. Consequently, when the cable is subjected to a tensile load, the load is not immediately taken up by the reinforcement. Instead, there is a lag, until the backlog of the reinforcement is taken up, which allows at least part of the load to be exerted on the optical fibre or fibres of the cable.
The present invention is based on the concept that the reinforcement should be prestressed and, more particularly, should comprise fibres which are held permanently under tension, so that, when in use, tensile loads are immediately taken up, without any lag, by the reinforcement.